Chapter 2: Geology of Ethiopia and the horn 2.1.Introduction: Geology is one of the Earth’s science which deals with the origin, composition, structure, history & materials of the Earth and the forces which act upon the Earth & inside the Earth’s surface Continental drift theory Australian Climatologist Alfred Wegener proposed the hypothesis that the continents were once assembled together as a supercontinent, called Pangaea. Pangaea was then split into Gondwanaland where Africa is a part and Laurasia; and later into smaller fragments over the last million years. These then drifted apart to form the present arrangement of continents. Wegener’s evidences of continents were once assembled together as a supercontinent were: 1.Fit of the continents: The opposing coastlines of continents often fit together. 2. Match of mountain belts, rock types: mountains in West Africa, North America, Greenland, and Western Europe match up. 3. Distribution of fossils: distribution of fossils on separate continents forms definite linked patterns if the continents are reassembled. 4. Paleo-climates: rocks formed 200 million years ago in India, Australia, South America, & southern Africa all exhibited evidence of continental glaciations. 2.2. The Geological Time Scale and Rocks age dating techniques Geological time scale It is a reference scale developed by scientist to describe the geology and history of life on Earth. It helps to understand the entire history of the earth into workable units It was constructed using the evidences collected from Field observations, Fossil records, Radioactive dating etc Several major incidences have happened in the history of the earth. Some of the major events mass extinctions appearance of new species or genera of life, mountain-building movements drifting of continents massive migration of life between land and water spreading of ocean floors widespread glaciations dominance of certain species Geological time divisions differ from each other in such characteristics Relative position of land and sea, occurrence of significant geological events (e.g. mass extinctions). Kind of climate &kind organisms that developed and existed during that geological time. Geological time is difficult to measure precisely. The first major boundary is defined by what was believed at the time to be the first appearance of life on earth. The geological time categories do not usually consist of a uniform length of time The geological time scale measures time on a scale involving four main units: 1. Eon The longest/ largest period of geological time (e,g Haddian, Achaean) 2. Eras It is combination of periods or subdivisions of Eons a) Precamberian Era (Archean & Proterozoic periods) b) Paleozoic Era (ancient life) It is the age of invertebrate Consists of Cambria, Ordovician, Silurian, Devonian, Mississippian Pennsylvanian & Permian periods c) Mesozoic Era (the middle life) Consist of Triassic, Jurassic & Cretaceous periods Age of reptiles d) Cenozoic Era (recent life) Age of mammals Divided into Tertiary, & quaternary periods 3. Periods- epochs are clumped together into larger units called periods 4. Epoch-is the smallest unit of time on the scale and encompasses a period of millions of years. Began (in Million Years) End (my) Major Events (million years ago) Quaternary 1.6 Present Major glaciers in North America and Europe (1.5) Tertiary 70 1.6 Cretaceous 146 70 Rocky Mountains (65), individual continents take shape. Dinosaurs extinct (65), western interior seaway and marine reptiles (144 – 65) Jurassic 208 146 Pangaea (one land mass) begins to break up (200) Triassic 225 208 First mammals and dinosaurs Permian 290 225 Greatest extinction on Earth (245) Pennsylvanian 322 290 First reptiles Mississippian 362 322 Coal-forming forests Devonian 408 362 First land animals and first forests (408) Silurian 439 408 Life invades land Ordovician 510 439 First fish appeared Cambria 600 510 Great diversity of marine invertebrates Proterozoic 2,500 600 Marine fossil invertebrates (600) Archean 4,500 2,500 Earliest fossils recorded (3,500), earliest rock formation (4,000) Precambrian Paleozoic oic Mesozoic Cenoz Era Period Rocks age dating techniques I. 1.Relative dating methods 1. Arranging events in order – it is possible to place a sequence of events in proper order. There are different principles in this method. A. Principle of superposition – in undisturbed pile of sediments those on the bottom where deposited first, followed in succession by the layers above them, ending with the youngest on the top. But, Unconformity: a gap in the rock sequence happens. B. Principle of original horizontality Sediments are deposited in approximately horizontal flat-lying layers. If layers are no longer horizontal, they must have undergone deformation after formation. C) Principle of cross cutting relationship Based on the fact that a rock must first exist before anything can happen to it. For example if dikes cutting through older rock, the country rock must be older than the dike. 2) Correlation geologists combine evidences from many localities. To do this, rocks of the same age from different localities must be matched in a process called correlation. If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. B. Absolute Dating/Radiometric techniques Techniques used to date materials like rocks by observing the abundance of naturally occurring radioactive isotope & it’s decay products It is based on the decay of natural, long-lived radioactive isotopes (parent) of certain elements, incorporated into a rock or mineral at the time of formation, into other stable isotopes (daughter). At the end of the period constituting one half-life, half of the original quantity of radioactive element has decayed; after another half-life, half of what was left is halved again, leaving one-fourth of the original, and so on. Terminologies Half life: The half-life is the amount of time it takes for one half of the initial amount of the parent, radioactive isotope, to decay to the daughter isotope/product . After one half-life passes, half of the remaining nuclei will decay in the next half-life. Then, half of that amount in turn decays in the following half-life. Therefore, the number of radioactive nuclei decreases from N to N / 2 in one half-life, to N / 4 in the next, to N / 8 in the next, and so on. Radioactive isotopes decay at a specific rate that is represented by the isotope’s decay constant or the number of disintegrations per year, which can be used to calculate its half life. Every radioactive element has its own half-life. E.g Carbon14 decays to Nitrogen14 with a half life of 5730 years So if we can measure the proportion of parent & daughter isotopes in rocks now, we can calculate when the rocks were formed t = 1 /λ ln (206Pb /238U +1 ) Daughter product The nuclear decay of radioactive isotopes is a process that behaves in a clocklike fashion and is thus a useful tool for determining the absolute age of rocks. 1) Uranium-Lead dating method It is one of the oldest & if done properly one of the most accurate. Uranium comes as two common Isotopes; U235 & U238. Both are radioactive & unstable, shedding nuclear particles in cascade that does not stop until they become lead (Pb). The two cascades are different, U235 becomes Pb 207 (half-life 704 million years), and U238 becomes Pb 206 (half-life 4.47 billion years). Lead atoms created by uranium decay are trapped in crystal and build up in concentration with time; helping us in dating. Uranium-Lead dating works mainly for metamorphic & igneous rock 2) Potassium-Argon Technique: The potassium-Argon (K-Ar) Isotope dating method is especially useful for determing the age of lavas/volcanic rocks. Potassium has one radioactive isotope (40K). Potassium -40 decay with half life of 1250 million years. Geologists are able to date entire rock samples in this way, because potassium-40 is abundant in micas, feldspars & hornblendes. 3) Carbon-14 Technique: Upon the organism’s death, carbon-14 begins to disintegrate at a known rate, and no further replacement of carbon from atmospheric carbon dioxide can take place. Carbon 14 decays with half life of about 5730 years by the emission of an electron, disintegrating to nitrogen 14. Carbon dating only works on fossils that used to be a lived. You can not carbon date a rock or a sedimentary layer 2.3. The Geologic forces/processes: Endogenic and Exogenic Forces I) Endogenic forces/processes 1) Diastrophism (slow geomorphic processes) It is large-scale deformation of Earth’s crust by natural processes, which leads to the formation of continents and ocean basins, mountain systems (fold Mt., Block Mt, ) plateaus, rift valleys, and lithospheric plate movement (plate tectonics) a) Orogenic/ mountain-forming processes act tangentially/laterally to the earth’s surface because of the impact of Tensional and compressional forces E.g Himalayan-Alpine orogeny. b) Epeirogenic processes/continent-forming movements vertical movement of land mass (rising/uplifting and sinking/subsidence) It is continental building process. E.g. East Africa 2) Sudden geomorphic processes Common at lithospheric plate borders as plate borders are quite unstable because of the pressure brought on by the mantle’s magma’s pushing and tugging/pulling. Examples- earthquakes and volcanoes II) Exogenic Forces a)Weathering Rocks breaking down physically (by e.g animals, plant, temp. change, abrasion etc) and chemically (by e.g by water, carbon dioxide, living things, acid rain etc) b) Erosion & deposition Natural forces like the wind, water, ice, and gravity transport the clay materials away. Weathering is the first stage of erosion. c) Mass Movements The shallow to deep columns of Earth’s materials creep, flow, slide, and fall as a result of slope movement. Gravitational attraction is exerted on the bedrock The landmass of Ethiopia, as elsewhere, is the result of the combined effect of endogenic and exogenic processes / 2.4. Geological Processes and the Resulting Landforms of Ethiopia and the Horn 1. Precambrian Era Due to its remoteness in time and the absence of well-preserved fossils, our knowledge of the events is limited. Nevertheless, some general description of the main geologic processes can be made. The major geologic event of the Precambrian include- a) Orogenesis Formation of huge mountain ranges mainly by folding. This was accompanied by intrusive igneous activity b) Denudation In b/n the orogenic periods & after the last orogenesis, there were long periods of denudation, which finally reduced these mountains to near-level (peneplained) rock surfaces. This “levelled” surface was later (Mesozoic & Cenozoic Eras) covered by younger rock formations. Therefore, in most parts of Ethiopia rocks belonging to this Era are found beneath all other rocks, forming the basement rocks (basement rock). Since, they had been subjected to pressure & heat from overlying weight earth movements (folding) intrusive igneous activity, the original rocks (both sedimentary and igneous) were altered into metamorphic rocks of varying stages of metamorphism. Since these same processes have allowed mineralization & crystal formation, the rocks are also collectively described as crystalline rocks. Precambrian rocks as surface rocks covering 25% of the land mass of the country; they are found exposed in the following areas: . A) Western Part: Gambella, Benishangul-Gumuz (Metekel and Asossa), western Gojjam, western Wellega, Illuababora, and Abay gorge. Baro river valley B) Southern Part: Guji, Southern Omo, and parts of southern Bale and Borena. Gamgofa C) Eastern part: Eastern Hararghe. D)North- Gondar, northern and central Tigray 2) The Paleozoic Era Geologic Processes (600million - 225 million years ago) The second oldest era & the resulting features were mostly geomorphic features. The life forms existed in this era include Fish, Amphibians & low forms of plants like Ferns & Mosses. The major geological events in Ethiopia during this Era were denudation. And peniplanation. The gigantic/huge/massive Mts that were formed during the Precambrian orogeny were subjected to intense & prolonged denudation and in turn reduced to a “peneplained” surface. Undulating plain with some residual features (inselbergs) here and there was formed. a) Inselberg or Volcanic plug in Girunmba (around Enjibara area, Ethiopia) b) Volcanic pluge (Bale area, Ethiopia) Because of the limited deposition within Ethiopia, rocks belonging to this Era are rare in the country. 3) The Mesozoic Era Geologic Processes (225-70 million years ago) The major geological events that took place in this era include: Mt. building Formation of sedimentary rock Existence of higher orders of life (flowering plants, birds & mamals) Epeirogenesis Epeirogenesis The process of alternative slow sinking & rising of the land mass & took place in the 3 consecutive periods of the era The whole present day Africa & Arabian land mass were affected by this process a) Triassic period (early period & lasted from 225-180 mil.yr. ago) The horn of Africa began to subside b/c of internal forces which let gradual invasion of sea water from Indian ocean (SE-direction) and slowly spreading NW in the late Triassic. This gradual invasion of sea water is called Transgression of sea water The invasion/transgression of sea resulted in deposition of the 1st & oldest sedimentary rock in Ethiopia, called Adigrat Sand stone (named after the place where it might have been 1st identified). Thus, the peniplained Precambrian rock surfaces started to be occupied by the sand carried by the shallow sea. Due to the tilting of the land mass and the direction of the invading sea the age & thickness of Adigrat sand stone varies from SE to NW direction (it is older & thicker towards SE-direction) As the depth of the sea increased another transported materials & remnants of marine life decaying & precipitating like shale, gypsum & lime were deposited. . Hence, Mesozoic rocks are considered to have the greatest potential for oil and gas deposits. b) Jurassic period (180-135 million years ago) The sinking of the land mass continued in this period The land surface of Ethiopia remained under sea water The water body was occupied by various life forms and hence the dead body of these organisms formed the second layer of sedimentary rock known as Hintalo limestone. c) Cretaceous period (135-70 million years ago) At the end of Jurassic & beginning of cretaceous the Horn of Africa & Arabia area were up lifted Following the uplifting of the land the regression of the sea began In Ethiopia the up lifting 1st began in the NW part and then it continued in the SE direction. In the process of uplifting and in turn regression/retreat of sea the 3rd & youngest sedimentary rock called upper sandstone formed. The age & thickness of upper sand stone varies from NW to SE direction (upper sand stone is older & thinner in NW direction and younger & thicker in the SE direction) In most parts of Ethiopia, the Mesozoic rocks are overlaid by the Cenozoic rocks. As surface rocks, these old marine sediments are extensively found in the Southeast lowlands central Tigray. gorges of Abay and Wabishebelle rivers 4) The Cenozoic Era Geologic Processes (70million years ago - Present) a) Tertiary period (70-2 mill. Yrs. Ago) The present land surfaces of Ethiopia were formed during this period The major geological events of this era includei) Uplifting of the Arabo-Ethiopian landmass & out pouring of large quantity of lava. ii) Formation of rift valley i) Uplifting of the Arabo-Ethiopian landmass & out pouring of large quantity of lava The uplifting of the Arabo-Ethiopian landmass was a continuation of the slow rise that began in the Mesozoic Era. The whole of the Arabo-Ethiopian landmass was pushed up in blocks as one mass. The greatest uplift was in central Ethiopia. This immense tectonic force also fractured the crust at many places and hence huge quantity of lava came out through these fractures The volcanic material is known as Trappean lava or Trap Series lava Lava out poured & spread widely & extensively and covered a large part of the Mesozoic sedimentary layer form the Ethiopian plateau surface and also the floor of the present-day Rift Valley (before its formation). The mass of lava was so immense, that it formed a thick layer of volcanic rocks on the plateau, Where the lava comes out through vents, huge volcanic rocks were piled up to form many & huge volcanic Mountains’ towering the flat basaltic plateau. Due to the out flow & extensive spread of the lava, the Ethiopian plateau has got flat & nearly horizontal surface. The flat & horizontal nature of the Ethiopian plateau is an indicator ofa) Pene plained Precambrian Mt. b) Smoothening of rough earlier rock surface by Mesozoic deposition c) Spreading of Trapean series lava over the Mesozoic sediments ii) Formation of the Rift valley Rift valleys are formed when land b/n two faults sinks down. The blocks on both sides of the valley from plateaus. As the tension widened the fractures, the central part of the landmass collapsed to form an extensive structural depression known as the Rift Valley. . Faulting-Rift valley formation Ethiopian rift valley Part of the longest rift valley in the world that extends from Syria in middle east to Mozabique in Africa (total length=7200, 5600km= in Africa &1,700 km in Eritrea and Ethiopia). The region is the most unstable part of the country ( hot springs, fumaroles, active volcanoes, geysers, & frequent earthquakes) Ethiopian RV is funnel shaped, and wider to the north & narrower to south. Its wider part is the Afar region for about 200-300km. The Red Sea, Gulf of Aden & East African System meet & form the triangular depression of the Afar where the Kobar Sink lies about 125/116 m.b.s.l The Afar depression (including the Gulf of Zula) was down-faulted allowing the Red Sea water to penetrate far inside. The area b/n the Danakil Depression & Red Sea was uplifted & form the Afar Block Mountains. Ethiopian RV with its NE to SW extension has effect on the physiography of Ethiopia i.e Divide/separeate Ethiopian plateau into two Arabian landmass from African landmass. It causes the formation of the Dead Sea, Red Sea and the Gulf of Aden troughs/channel. It creates basins & fault depressions on which the RV lakes are formed. b) Quaternary period (2 million-yrs. ago--present day) The major geological events observed in this period were volcanic activities marked climatic change appearance of modern man & abundance of large mammals a) Volcanic activity The quaternary volcanics are called Aden volcanic or Aden series. Recent volcanic activities after the formation of RV. Aden volcanic & recent faulting are extensively developed in the Afar region. The eruption was also a common character of the northern section of lakes region of Ethiopia & even some eruption took place in the last few hundred years The basic visible volcanic features include volcanic cones volcanic hills Calderas with huge depressions Extensive lava sheets Lava ridges Thermal springs, geyser, fumaroles Volcanic ash Volcanic cones Volcanic Ash layer in the rift valley rim of Wunche town b) Marked climatic change It was a period of extensively heavy rain fall. Rain known as the pluvial in Africa. In the middle & higher latitudes it was period of an ice cover & glaciation. Due to the pluvial rain in Africa Ethiopian plateau were altered by heavy erosion. There was an excessive surface flow & enlargement in size & number of rivers, lakes & marsh areas. Some lakes regions of Ethiopia were merged together. Example Ziway, Langano & Shalla Awassa & Shalla (East of Awassa lake) Chamo & Abaya Abbe & near by smaller lakes After the pluvial period when the climate was changed to warm & dry condition dryness which increase evapotranspiration Rivers & rift valley lakes decrease in size & merged lakes were left apart Areas covered by water bodies in pluvial period were exposed & left observable their deposits. These are called the quaternary deposit- The quaternary deposit include- a) Lacustrine deposit (lakes bed) b) Fluvial deposit (deposits on banks of river) c) Glacio-fluvial deposits (deposits on higher altitudes & Mt.) d) Coastal deposits (deposits transported by wind) e) Marine deposits (deposits around & on the floor of Seas)- include the salt deposits at Danakil depression 2.5. Rock and Mineral Resources of Ethiopia Rocks a) Igneous rocks-formed from molten rock/magma. It is dominantly found in central parts of Ethiopia b) Sedimentary rocks- formed by deposition of sediments e.g lime stone, coal, rock salt etc. Mainly found in east & southeast coastal areas of Ethiopia. c) Metamorphic rocks- modified/changed rocks from igneous/sedimentary rock even from metamorphic rock itself by heat & pressure effect. Mainly found in south & north west Ethiopia In Ethiopia there is long mining tradition i.e gold production and utilization, which has become part of Ethiopia’s history, tradition and folklore mining and working of iron for the manufacture of tools, utensils and weapons use of salt and salt-bar However, presently mineral production from Ethiopia has been negligible by World standards. Geological surveys proved that Ethiopia has abundant mineral resources of metals and precious metals, coal, and industrial minerals. The occurrence of metallic minerals in Ethiopia is associated with the Precambrian rocks. Minerals The occurrence of metallic minerals in Ethiopia is associated with the Precambrian rocks. Ethiopia has abundant mineral resources of metals and precious metals, coal, and industrial minerals. Gold Gold has been mined in Ethiopia for quite long time, mainly from Benishangul- Gumuz (Metekel) and Adola. Operating mines produce gold from primary sources in such localities as Dermidama, Sakoro and Lega-dembi. Mechanized alluvial working is confined to the state-operated gold field of Adola. Secondary gold deposits are common in the following localities Adola, Murmur Basin, Shakiso, Awata Basin, Dawa Basin, Ghenale Basin, Ujama Basin, Makanisa (Guba and Wombera), Kaffa. In Gambella and Illuababora (Akobo River), in Sidama (Wondo), Borena (Negele-Yabelo area) and in Benishangul-Gumuz (Sherkole), west Wellega, Mengi-Tumat-Shangul areas to the Sudanese border, and the drainage of the Didessa and Birbir. Platinum The Yubdo area in Wellega, is the only active Ethiopian Platinum mine. Platinum occurrences have been reported from Delatti in Wellega, and the valley of Demi-Denissa and Bone Rivers as well as Tullu Mountain area in Sidama. Tantalum Significant deposit of tantalum and niobium is found in southern Ethiopia. It occurs in Adola area where Kenticha Tantalum mine with resources of more than 17,000 metric tons of world class ore reserve is found. Coal Extensive lignite deposits in Ethiopia are found in Nedjo (Wellega), and in small amounts in Chilga (Gonder) are found in the sedimentary formations laid in between Trapean lava. Gemstones Gemstones, including amethyst, aquamarine, emerald, garnet, opal, peridot, sapphire, and tourmaline occur in many parts of Ethiopia, mainly in Amhara and Oromia Regional States. Quality Opal was first discovered by local people in Wadla and Dalanta woredas, North Welloin Amhara Regional State. Potash The potash reserve in the Danakil (Dallol Depression) of the Afar region is believed to be significant. Gypsum and Anhydrite A limited amount of gypsum is produced for domestic consumption in Ethiopia, mainly for the cement industry, but very large deposits are known to occur in sedimentary formations of the Red Sea coastal area, Danakil Depression, Ogaden, Shewa, Gojjam, Tigray, and Hararghe. . Marble Crystalline limestone is widespread in the basement rocks of Ethiopia. Marble has been quarried in such localities as west of Mekelle and south of Adwa in Tigray. In the east in Galetti, Soka, Ramis, Rochelle, Kumi and other valleys of Chercher Mountain in West Hararghe. In the northwestern also in areas built of Precambrian schist in Gonder, and the Dabus River and other neighboring river basins in Benishangul-Gumuz and Gojjam. Construction stones Basalt, granite, limestone and sandstone are important building stones. For the surfacing of roads and compaction, basalt, scoria and other volcanic rocks are extensively used. Mesozoic limestone is an important raw material for cement and chalk production. The earlier cement works at Dire Dawa and the recent ones at Muger Valley, Abay gorge (Dejen), Tigray (Messebo) are using similar raw materials from these rock formations. Clay Alluvial clay deposits for bricks and tile, pottery and pipe industry occur in Adola, Abay gorge, and the Rift Valley lakes region. Ceramic clay for the production of glasses, plates, bricks is found at Ambo and Adola. Tabor ceramic industry in Hawassa gets most of its raw materials from local sources Mineral Potential Sites of Ethiopia 1. The Western and South-western-greenstone belt: They contain various minerals: primary gold occurrences Yubdo Platinum, Base metals Molybdenite and the iron deposits Marble, Akobo and Asosa placer gold deposits and etc. 2. The Southern greenstone belt: It is known as the Adola belt, Comprises the primary gold deposits and occurrences of Lega-dembi, Sakaro, Wellena, Kumudu, Megado-Serdo, Dawa Digati, Moyale and Ababa River;the columbo-tantalite of kenticha and Meleka, andthe Adola nickel deposit and other industrial minerals. 3. The Northern greenstone belt This belt comprises of the primary gold occurrences of Terakemti, Adi-Zeresenay, and Nirague. The base metals of Terer, Tsehafiemba and other parts of Tigray, Placer gold occurrences of Tigray.
